1. Field of the Invention
The present invention relates to a laser irradiation apparatus (an apparatus including a laser and an optical system for guiding a laser beam emitted from the laser to an irradiation object) for carrying out annealing to, for example, a semiconductor material homogeneously and effectively.
2. Related Art
In recent years, a technique for manufacturing a thin film transistor (hereinafter referred to as a TFT) over a substrate has been developed drastically, and application to an active matrix display device has been advanced. In particular, a TFT formed using a poly-crystalline semiconductor film is superior in field-effect mobility to a TFT formed using a conventional amorphous semiconductor film, and therefore high-speed operation becomes possible when the TFT is formed using the poly-crystalline semiconductor film. For this reason, it has been tried that a driver circuit, which has been conventionally provided outside a substrate, is formed over the same substrate as a pixel and that the driver circuit controls the pixel.
With the increase in the demand of semiconductor devices, it has been required to manufacture the semiconductor devices in shorter time at lower temperature. As a substrate used in a semiconductor device, a glass substrate, which is less expensive than a quartz substrate, is often used. Although the glass substrate is sensitive to heat and easy to deform due to the heat, a TFT using a poly-crystalline semiconductor film formed over a glass substrate can be obtained easily by crystallizing a semiconductor film formed over the glass substrate at low temperature according to laser annealing.
Moreover, compared with an annealing method using radiant heat or conductive heat, the laser annealing has advantages that processing time can be drastically shortened and a semiconductor film over a substrate is heated selectively and locally so that almost no thermal damage is given to the substrate.
As laser oscillators used in the laser annealing, there are a pulsed laser oscillator and a continuous wave laser oscillator according to the oscillation method. In recent years, it has been known that the crystal grain formed in the semiconductor film becomes larger when using the continuous wave laser oscillator such as an Ar laser or a YVO4 laser than when using the pulsed laser oscillator such as an excimer laser in crystallizing the semiconductor film. When the crystal grain in the semiconductor film becomes larger, the number of crystal grain boundaries in the channel region of the TFT formed using the semiconductor film decreases, and the mobility becomes higher so that more sophisticated devices can be developed. For this reason, the continuous wave laser oscillator is attracting attention.
However, the laser annealing with a continuous wave laser oscillator has a problem in that an annealed state of an irradiation surface becomes inhomogeneous. This is because a laser beam emitted from a continuous wave laser oscillator has Gaussian energy distribution in which the energy is attenuated from the center toward the ends. Accordingly, homogeneous annealing is difficult to achieve.
An apparatus shown in FIG. 6 is known as an apparatus for shaping a laser beam into a linear spot and homogenizing the energy of the laser beam on the irradiation surface. The laser irradiation apparatus has a plurality of cylindrical lens arrays and the like. A laser beam emitted from a laser oscillator 1 is divided into a plurality of beams and condensed by cylindrical lens arrays 2 to 6, and then transmitted through convex cylindrical lenses 5 and 6. The cylindrical lenses 5 and 6 are arranged so that their generating lines intersect with each other. After that, the laser beams are reflected by a mirror 7, and then condensed on an irradiation surface 9 by a doublet cylindrical lens 8 in such a way that the laser beams are combined into one linear beam on the irradiation surface 9. The doublet cylindrical lens 8 consists of two cylindrical lenses.
By performing laser irradiation while displacing the linear beam in its minor-axis direction, the entire surface of an amorphous semiconductor is annealed so that crystallization, enhancement of crystallinity, and activation of an impurity element can be performed.
However, since the conventional laser irradiation apparatus needs to use a plurality of expensive cylindrical lens arrays and to arrange them so as to form a desired linear beam as described above, the apparatus has a problem in that the size and cost of the apparatus increases. Further, the laser annealing is performed by shaping the linear beam into a desired size. When the size of the linear beam is changed in laser annealing, it is necessary to rearrange the optical system or to replace an optical component. Therefore, optical alignment is required every time the size of the linear beam is changed, which consumes a large amount of time. This results in the low throughput (see Reference 1: Japanese Patent Application Laid-Open No: 2003-257885).
The present inventor has already suggested a laser irradiation apparatus which is compact and less expensive to overcome the problems of the conventional laser irradiation apparatus. FIG. 7 illustrates the laser irradiation apparatus suggested by the present inventor, including a diffractive optical element, which is an element for forming a rectangular beam spot with homogeneous energy distribution.
Specifically, in FIG. 7, a laser beam emitted from a laser oscillator 11 is reflected by a mirror 12, and is then shaped into a rectangular beam spot 14 with homogeneous energy distribution by a diffractive optical element 13. An irradiation surface 15 is provided at the position where the beam 14 with homogeneous energy distribution forms an image (See Reference 1: Japanese Patent Document Laid-Open No. 2003-257885).
However, the above laser irradiation apparatus still has a problem of inhomogeneous energy distribution at the end portions of the rectangular beam. Moreover, since it is necessary to replace the diffractive optical element or other lenses or to rearrange the optical elements in order to change the size of the beam, the optical alignment takes much time, thereby resulting in the low throughput.